Swarm describes a behaviour of an aggregate (Aggregation) of animals of similar size and body orientation, often moving en masse in the same direction. "Swarming" is a general term that can be applied to any animal that swarms. The term can be applied to insects, birds, fish, various microorganisms such as bacteria, and people. The term applies particularly to insects. "Flocking" is the term usually used for swarming behaviour in birds, while shoaling or schooling (Shoaling and schooling) refers to swarming behaviour in fish. The swarm size (Group size measures) is a major parameter of a swarm.